1. Field of the Invention
The present invention relates to an ultrasonic Doppler diagnostic equipment and an image data generation method wherein the flow velocity information of a blood flow within a living body, the movement information of a tissue, etc. are measured by utilizing the Doppler effect of ultrasounds.
2. Description of the Related Art
An ultrasonic diagnostic equipment is such that ultrasonic pulses which have been generated from piezoelectric transducers built in an ultrasonic probe are radiated into a patient, and that ultrasonic reflected waves generated by the difference of the acoustic impedance of a patient tissue are received by the piezoelectric transducers and then displayed on a monitor. Such a diagnostic method is extensively employed for the functional diagnoses and morphological diagnoses of the various internal organs of the living body because a two-dimensional image can be easily observed in real time by the simple operation of merely bringing the ultrasonic probe into touch with the surface of the body. An ultrasonic diagnostic method which obtains in-vivo information on the basis of reflected waves from a tissue or blood corpuscles within a living body has made rapid progress owing to the great technological developments of two methods; an ultrasonic pulse echo method and an ultrasonic Doppler method. A B-mode image and a color Doppler image which are obtained using the technologies, are indispensable to ultrasonic image diagnoses of today.
On the other hand, a Doppler spectrum method is a method which quantitatively and precisely obtains a blood flow velocity at any desired position of a patient. In the Doppler spectrum method, ultrasounds are transmitted to and received from the same part of the patient at regular intervals a plurality of times, and ultrasonic reflected waves from mobile reflectors such as blood corpuscles are subjected to orthogonal phase detection by employing a reference signal whose frequency is substantially equal to the resonance frequency of piezoelectric transducers used for the ultrasound transmissions and receptions, thereby to detect Doppler signals. Herein, a Doppler spectrum is calculated in such a way that Doppler signals at the desired part is extracted from among the detected Doppler signals by a range gate, and that the extracted Doppler signals are further subjected to an FFT (Fast-Fourier-Transform) analysis.
Doppler spectra are continuously calculated for Doppler signals obtained from the desired part of the patient by such steps, and the plurality of calculated Doppler spectra are successively arrayed, thereby to generate so-called “Doppler spectral image data”. By the way, in general, the setting of the range gate is performed under the observation of a B-mode image in order to confirm that the range gate is precisely set at the desired observation part in the patient. On this occasion, the position of the range gate is displayed on the B-mode image.
An example of a Doppler spectral image is shown in FIG. 1. (a) on the left side of FIG. 1 shows a Doppler spectrum obtained by an FFT analysis, in which the axis of ordinates represents a Doppler frequency, while the axis of abscissas represents the magnitude (termed “power value” below) of the spectrum. Besides, (b) on the right side of FIG. 1 shows the temporal variation of the Doppler spectrum, in which the axis of ordinates is set at the Doppler frequency, while the axis of abscissas is set at time. The power of the spectrum is expressed in terms of an intensity or brightness.
Meanwhile, it has heretofore been known that random interferences occur among reflected waves from mobile reflectors within a patient, and that interference noise (speckle noise) is consequently generated in a Doppler spectral image. More specifically, as shown in (a) of FIG. 1, a calculated Doppler spectrum 151 (solid line) exhibits unevenness ascribable to the interference noise, with respect to a true Doppler spectrum 152 (broken line). Therefore, a discontinuous pattern ascribable to the influence of the interference noise is displayed also in (b) of FIG. 1 showing the temporal variation of the Doppler spectrum, and it becomes difficult to precisely measure the temporal variations of a blood flow velocity, etc. The influence of such interference noise is conspicuous in a case where the power value, namely, S/N ratio of a spectral component is small. Accordingly, in a case where the maximum blood flow velocity is measured by tracing the maximum frequency component 153 of the spectrum, precise automatic tracing or manual tracing becomes difficult. As another problem, especially in the case of the manual tracing, a long time is expended on the tracing, to increase a burden on an operator who performs the tracing.
In order to cope with such problems, there has been proposed a method wherein the interference noise is reduced by taking moving averages in a time direction in units of the individual frequency components of a Doppler spectrum (refer to, for example, JP-A-6-327672).
According to the method stated in the patent document, the influence of the interference noise is relieved, so that the edge parts of the maximum frequency component, etc. in the Doppler spectrum can be displayed continuously and smoothly, and a visuality in the case of performing the tracing is enhanced. Since, however, the moving average in each individual frequency component of the Doppler spectrum needs to be taken for a comparatively long time period for the purpose of attaining such an advantage, a sharpness on a Doppler spectral image degrades drastically. In particular, a subtle variation in the time direction or in a frequency direction, at a near-mean-frequency component having a large power value, has heretofore been deemed effective as diagnostic information, but the method in the patent document becomes difficult of sharply displaying the temporal variation of such a near-mean-frequency component.